Electron acceptor substituents

Different behavior is expected for X = N-R Figs. 3 and 4) the equilibrium should be shifted toward 1 when an electron donor substituent (a = -l) is present (in any position) and toward form 2 when electron acceptor substituents are present. Ii has been shown chat a simultaneous... 

Electron-donor and electron-acceptor substituents selectively interact with different ring orbitals. Compare the HOMO and LUMO of azobenzene with the corresponding orbitals of the two substituted molecules. Which orbitals show signficant substituent contributions What are the nature of these contributions, bonding or antibonding Try to relate this to the effect which the substituents have on orbital energies and on the HOMO-LUMO gap in azobenzene. 

Diethyl arylphosphonates were synthesized by reactions of diethyl phosphonate with aryl iodides or bromides containing electron-donor or electron-acceptor substituents in the aromatic ring in aqueous MeCN or neat H2O in the presence of Pd complexes with water-soluble ligands.34 For example, MeCN/H20 (1 mL), PhBr (8.2 mmol), and Ph2P(C6H4S03Na-m) (approximately 0.4 mmol) were successively... 

Comproportionation equilibrium constants for Equation 9.3 between dications and neutral molecules of carotenoids were determined from the SEEPR measurements. It was confirmed that the oxidation of the carotenoids produced n-radical cations (Equations 9.1 and 9.3), dications (Equation 9.2), cations (Equation 9.4), and neutral ir-radicals (Equations 9.5 and 9.6) upon reduction of the cations. It was found that carotenoids with strong electron acceptor substituents like canthaxanthin exhibit large values of Kcom, on the order of 103, while carotenoids with electron donor substituents like (J-carotene exhibit Kcom, on the order of 1. Thus, upon oxidation 96% radical cations are formed for canthaxanthin, while 99.7% dications are formed for P-carotene. This is the reason that strong EPR signals in solution are observed during the electrochemical oxidation of canthaxanthin. 

Boryl-borate tautomerism This type of tautomerism is observed for boryloxyalkylphosphines (101), (102) possessing a hydroxyalkyl group and electron-acceptor substituents at the a-carbon atom [Eq. (88)] (851ZV1102 90IZV1133). Mutual transformations take place rapidly and the equivalency of methyne protons in NMR spectra serves as evidence of tautomerism. In the crystalline state these compounds exist in form A. The considerable influence of the nature of the solvent (-4 ppm in DMFA - 10 ppm in C6H6) on the position of the signal in the MP NMR spectrum provides evidence in favor of tautomerism. 

When (5(15N) values for the previously used series of 13 para-substituted anilines were measured in acetone42, a significantly weaker hydrogen-bond acceptor solvent than DMSO, a smaller shift dependence on para TT-electron-acceptor substituent solvation (SSAR) effects in acetone was observed (Table 10). This reduction42 was expressed by the (rather unsatisfactory) forms, 4 and 5. 

SCHEME 1. Main reaction pathways during interaction of nitrosoarenes with thiols (RA denotes electron acceptor substituent and RD electron donor... 

The reaction of Scheme 4.10 yields only products of ortho and para substitutions the meta isomer is lacking. If it were a standard radical substitution, the meta-isomer would obviously be formed in a certain amount (i.e., in the same amount as that for ortho-substituted product). Introduction of electron-acceptor substituents enhances stability of the substrate to oxidation and prevents electron transfer to benzoyloxy radical. As a result, phenylation takes place instead of benzoyloxylation, and the phenyl radical enters into any free position. 

Let us now consider the formation of aryl iodides from aryl diazonium salts and potassium iodide in methanol (Singh and Kumar 1972a, 1972b). Electron-donor substituents decelerate the process as compared with benzene diazonium (the substituent is hydrogen), whereas electron acceptor substituents accelerate it. Oxygen inhibits the reaction, and photoirradiation speeds it up. As the authors pointed out, in the case of 4-nitrobenzene diazonium, the reaction leads not only to 4-iodonitrobenzene but also to nitrobenzene, elemental iodine, and formaldehyde. All of these facts support the following sequence of events ... 

Feng et al. (1986) performed quantum-chemical calculations of aromatic nitration. The resnlts they obtained were in good accordance with the IPs of N02 and benzene and its derivatives. The radical-pair recombination mechanism is favored for nitration whenever the IP of an aromatic molecule is much less than that of N02. According to calculations, nitration of toluene and xylene with N02 most probably proceeds according to ion-radical mechanism. Nitration of nitrobenzene and benzene derivatives with electron-acceptor substituents can proceed through the classical polar mechanism only. As for benzene, both mechanisms (ion-radical and polar) are possible. Substituents that raise the IP of an aromatic molecule to a value higher than that of N02 prevent the formation of this radical pair (one-electron transfer appears to be forbidden). This forces the classical mechanism to take place. It shonld be nnderlined that a solvent plays the decisive role in nitration. 

While lanthanide triflates have been demonstrated to promote the reaction of indoles with imines <99SL498>, Johannsen has developed a new synthesis of optically active p-indolyl N-tosyl a-amino acids 110 via the enantioselective addition of A-tosylimnio esters of ethyl glyoxylate 109 to indoles 108 bearing both electron-donor and electron-acceptor substituents at C-5 using 1-5 mol% of a chiral copper(I)-Tol-BINAP catalyst <99CC2233>. 

Because radicals are known to be stabilized by electron donor and by electron acceptor substituents, it has been proposed that radicals enjoy particular stabilization when they are substituted simultaneously by a donor and an... 

Contrary to amines, some structural variations of the diglycidyl ethers of bisphenols, such as the position of the glycidyl groups in the aromatic ring or the presence of either electron-donor or electron-acceptor substituents, have little effect on their reactivity with amines 103 -105>. o-Diglycidyl ethers the rate constant of which is higher by a factor of 5 than that of m- and p-isomers are an exception 104). These data are presented in Tables 7 and 8. 

For pyrroles with electron acceptor substituents in the 1-position electrophilic substitution with soft electrophiles can be frontier orbital controlled and occur at the 2-position, whereas electrophilic substitution with hard electrophiles can be charge controlled and occur at the 3-position. 

For electron acceptor substituents such as N02, CN and COR in the 2-position, position 4 is least deactivated by the substituent, but position 5 is most activated by the ring heteroatoms. In fact, such 2-substituted furans give exclusively 5-substitution, whereas for analogous thiophenes and especially pyrroles increasing amounts of 4-substitution occur. The harder the electrophile, the greater the tendency to 4-substitution. 

Photochemical substitution reactions can however follow other pathways than the concerted one which is the rule in the ground state processes. The orientation effects of electron donor and electron acceptor substituents are based on the model of a transition state of a complex which implies a concerted reaction (Figure 4.65). 

More recently, prodan has also been used to eliminate the charge effects observed with ANS (Haskard and Li-Chan, 1998). Prodan has both electron-donor and electron-acceptor substituents within the molecule, and is thus much easier than DPH to dissolve in more polar solvents (e.g., methanol). As a result, a simpler procedure than that of DPH was feasible. 

The proposed exciplex orientation, suggested to dictate the regioselectivity of the photoaddition, is consistent with the orientation of the dipolar attraction between the electronically n,7r excited triplet enone and the ground state alkene. Generally, electron acceptor substituents on the alkene provide preferential formation of the head-to-head (H,H) products, whereas electron donor substituents provide preferential formation of the head-to-tail (H,T) photoproducts55 (Scheme 16). 

Introduction of the electron-acceptor substituents enhances the stability of the substrate to oxidation and prevents electron transfer to the benzoyloxy radical. As a result, phenylation takes place instead of benzoyloxylation, and the phenyl radical enters into any free position. 

Ion radicals play a role as mediators in these two-electron transfers. Each one-electron step achieves a maximal rate, and both rate constants become close. Coulombic repulsion of positive (or negative) charges makes the double-charged ion formation difficult. Therefore, donors (or acceptors) are preferable for which some possibility exists to disperse the charge. Extension of the 77-system reduces intramolecular coulombic repulsion in the dianion state. Electron-donor (or electron-acceptor) substituents should be located at diametrically opposite sites of the molecule. Examples are ll,ll,12,12-tetracyano-9, 10-an-thraquinodimethane, TCNQ, DCNQI, and tetracyanobenzene. 

This regioselectivity results from the capto dative effect of intermediate radicals which are stabilized by the presence of one electron-donor and one electron-acceptor substituents [2]. The stereoselectivity of a-bromosubstitution is in agreement with the... 

---